Rotating mills are used in grinding and self-grinding of various ores. For example, in Semi-Autogenous Grinding (SAG) mills, a rotating drum throws steel balls in a cascading motion to impact and break up larger rocks. This attrition between grinding balls and ore particles causes grinding of finer particles. In self-grinding mills, larger rocks of ore cause impact breakage of other rocks and compressive grinding of finer particles. This grinding action is generally aided by slurries of grinding liquids. The inside of the mill drum is lined with lifting plates to lift the ore, slurry and balls inside the mill. The inside of the mill drum is also lined with a disposable cast metal liner, often with a rubber intermediate liner.
The mill liner is secured to the interior of the drum by an array of mill liner bolts inserted into non-round countersunk through-hole bores in the liner, passing through collocated bores in the rubber liner and drum. The mill liner bolts generally include a threaded shank and a non-round head defining load-bearing chamfered or tapered surfaces roughly corresponding to the taper of the countersunk bores in the mill liner. The mill liner bolts are loaded by tightening of a nut threaded onto the mill liner bolt shank threads extending beyond the exterior of the drum.
In some installations an exterior sealing plate is positioned between the drum exterior and the nut to provide and exterior seal to mitigate escape of slurry liquids through the mounting bores. Persistent flow of grinding fluids through the bores and between the liner, intermediate rubber liner, and/or drum can lead to scouring or racing of metal surfaces. This racing can lead to premature wear and failure of the liner, drum shell, shell ends, and other affected structures.
Typically, the bolt holes in the liners are cast approximately ⅛″ larger, all around the bolt head, for a standard 1½″ liner bolt. Point loads can be created by clearance and manufacturing tolerances or debris between the liner, which is cast, and the radiused, tapered, load bearing area of the mill liner bolt head, which is forged. Poor load contact can cause early embedment with subsequent loss of bolt clamp load. In many instances, the bolts may not establish enough surface area contact to remain tight and need to be periodically tightened through the life of the liners. Often, loose bolts fatigue and break with expensive consequences if the liners become detached. Running a mill with loose bolts can also cause bolt holes in the mill drum shell to become enlarged, which encourages further loosening and leaking problems eventually leading to expensive repair or replacement of the mill drum shell.
Point loads from debris or misalignment of load-bearing surfaces and loosening of the mill liner bolts can also cause premature failure of the liner, e.g., due to partial pull-through of the mill liner bolt head through the cast liner. In particular, loading of the mill liner bolt, compression of the intermediate rubber liner, and end compaction of the mill liner bolt head by ore or mill balls can generate significant stresses around the bore in the mill liner. Premature failure of the liner and mill liner bolts can lead to significant costly downtime as bolts are retightened or as the bolts and liner are replaced.
Accordingly, improvements are sought in mitigating racing, point loading and pull-through in mill liner installations.